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ISSUE 4 - SATURDAY 31 MAY 2014 - YEAR 1 
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Interfaces in High Voltage Engineering: 

A Most Important Question for Conventional 
Solid Insulating Materials as well as for 
Nanocomposite Polymers 



M.G. Danikas, R. Sarathi 



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Editorial Team 

# Electrical Engineering 

# High Voltage Engineering # Insulating Materials 
Professor Michael Danikas, mdanikas@ee.duth.gr 
EECE, Democritus University of Thrace, Greece 

# Electrical Machines # Renewable Energies # Electric Vehicles 
Assistant Professor Athanasios Karlis, akarlis@ee.duth.gr 
EECE, Democritus University of Thrace, Greece 

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ISSUE 7 - YEAR 2 



ELECTRICAL ENGINEERING 



EXPERIMENTATION 



M.G. DANIKAS, R. SARATHI 



Interfaces in High Voltage Engineering: 

A Most Important Question for Conventional 
Solid Insulating Materials as well as for 
Nanocomposite Polymers 

M.G. Danikas, R. Sarathi * 

Power Systems Laboratory, Department of Electrical and 

Computer Engineering, Democritus University of Thrace, 

Xanthi, Greece [l] 

Department of Electrical Engineering, 

Indian Institute of Technology Madras, Chennai, India [2] 

Abstract 

Interfaces consist a most important part of conventional 
insulating systems at high voltages. They are considered to 
be problem areas which have to be dealt with. Numerous pub- 
lications have contributed in rendering the mechanisms of 
interfaces understandable. On the other hand, interfaces in 
nanocomposite polymers seem to function in an entirely dif- 
ferent manner from that in conventional insulating systems. 
The present paper reviews past work on both the conventional 
insulations and in nanocomposites. Differences regarding the 
interfaces are mentioned and discussed. Whereas interfaces 
in conventional insulating systems are to be avoided, inter- 
faces in nanocomposite polymers seem to be desirable - at 
least - up to a certain percentage of nanoparticles in the 
base polymer. Although things are better understood in con- 
ventional insulating materials, more work has to be per- 
formed in order to clarify several aspects, such as space 
charges and electrical trees emanating from enclosed cavi- 
ties. Needless to say that much more work has to be done in 
nanocomposites w.r.t. their modeling and possible explana- 
tions of the surprising performance of interfaces, a perfor- 
mance that deviates strongly from the performance in classi- 
cal insulating materials. 



Keywords 

Breakdown, breakdown strength, conventional insulating 
materials/systems, nanocomposite polymers, partial dischar- 
ges, electrical treeing/trees 



Introduction 

With some sort of exag- 
geration, a well known pro- 
fessor said once that "the 
problems of high voltage in- 
sulations are problems of in- 
terfaces" [1] . Interfaces re- 
sult when there are two dif- 
ferent insulating materials 
next to each other or when an 
insulating material meets a 
conductor. An interface may 
become, e.g., the source of 
partial discharges or even the 
cause for a complete break- 
down of an insulating system. 
When two insulating materials 
of different dielectric con- 
stants have a common inter- 
face, then the material with 
the lowest dielectric con- 
stant will undergo the more 
intense electric stressing 
[2] . Depending also on other 
parameters, such stressing 
may result in a gradual dete- 
rioration of the insulating 
system and consequently in a 
complete breakdown [3] . 

In this paper, the ques- 
tions raised by the existence 
of interfaces in high-voltage 
insulating systems are dis- 
cussed. Interfaces play an 
important role in determining 
the robustness of an insulat- 
ing system, when conventional 
insulating materials are 
used. Interfaces play also a 
vital role in insulating sys- 
tems with nanocomposite poly- 
mers. Their functioning, how- 
ever, is of another nature. 



This short review is by no 
means exhaustive since the 
topic of interfaces is a vast 
one. It is the aim of the au- 
thors to give the gist of the 
problems and questions the 
researchers may face and to 
offer some comments. In the 
context of this paper, the 
terms "insulating material" 
and "dielectric material" or 
simply "dielectric" are some- 
times interchanged meaning 
the same thing. 

Interfaces in Conventional 

Insulating Materials 

Dielectric breakdown in 
insulating materials depends 
on electrode configuration, 
insulating material thick- 
ness, electrode materials, 
presence of microcavities, 
temperature, pressure, nature 
and morphology of material 
under test, type of applied 
voltage, damage path (surface 
or volume) [4] . Various di- 
electric breakdown theories 
have been put forward [5]- 
[13] . No matter whether the 
proposed theory was based on 
cumulative impact ionization 
by electrons - creating thus 
positive space charges which 
distort the field distribu- 
tion and weaken the dielec- 
tric [5] -, on the notion of 
"intrinsic breakdown" - ac- 
cording to which a large num- 
ber of electrons trapped in 
energy levels due to lattice 
imperfections can transfer 



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INTERFACES IN HIGH VOLTAGE ENGINEERING: A MOST IMPORTANT QUESTION 



M.G. DANIKAS, R. SARATHI 



energy to the lattice vibra- 
tions [6] - [8] , on the "40 ge- 
nerations avalanche theory" 

[9] , on the importance of spa- 
ce charges which modify the 
local electric field value 

[10] , on the right assumption 
that the breakdown is a prop- 
erty of the dielectric mate- 
rial plus its electrode sys- 
tem [11] or on the theory 
based on the ionizing elec- 
trons and the hole traps 
[12], [13], the fact remains 
that all the above mentioned 
phenomena result from elec- 
tric field intensifications, 
i.e. from either electrode 
imperfections or mismatch of 
dielectric materials. This 
brings us to the point men- 
tioned in the Introduction of 
the present paper: that in- 
terfaces may create the con- 
ditions which may cause elec- 
tric field intensifications. 

The subject of partial 
discharges (PD) which may en- 
sue because of electric field 
intensifications and/or be- 
cause of gas (or foreign par- 
ticle) inclusions in a solid 
dielectric material, has been 
studied by Mason in his fun- 
damental publications [14]- 
[18] . Having in mind all the 
above, it is fitting to say 
that interfaces - created ei- 
ther by a mismatch of dielec- 
tric materials or because of 
intrusion of foreign parti- 
cles and/or air cavities in 
the insulating material under 



question -, are the problem 
areas of an insulating sys- 
tem . 

As was pointed out quite 
early [19], interfaces play a 
most significant role in the 
discharge and breakdown pro- 
cesses: even if an insulation 
does not contain any cavi- 
ties, at a sufficient stress 
"some event" releases gas 
with the subsequent formation 
of a cavity. The cavity is 
occupied by a gas discharge 
which increases the rate of 
gas formation with the subse- 
quent growth of both the cav- 
ity size and the discharge 
intensity. The importance of 
the differing nature of in- 
terfaces was also stressed in 
another publication, where it 
was pointed out that damage 
in internal cavities in poly- 
ethylene is little compared 
to the electrode adjacent ca- 
vities of the same dimensions 
and tested under the same ex- 
perimental conditions [20] . 
It is evident that in [20], 
interfaces between polyethy- 
lene and gas were compared 
with interfaces between poly- 
ethylene and electrodes, and 
the latter were found to be 
more dangerous and deleteri- 
ous to the insulating mate- 
rial. On the same lines, 
Kreuger showed that with PVC- 
insulated cables, the number 
of discharges increased with 
increasing electric stress in 
the dielectric [21] . In yet 



another paper, it was indi- 
cated that the nature of in- 
ternal discharges was greatly 
affected by the assembly of 
the electrode system and the 
adhesion of the insulating 
tapes [22] . Discharges always 
start in the electrically 
weaker insulating material, 
as was commented in previous 
works [23] - [25] . 

Needless to say that phe- 
nomena related to PD, such as 
electrical treeing, are also 
closely connected to the mis- 
match of the dielectric con- 
stants of insulating materi- 
als and/or to the existence 
of gas cavities in their vo- 
lume. Earlier papers indi- 
cated that the treeing phe- 
nomenon in polyethylene ca- 
bles started from both inner 
and outer surfaces and also 
from solid particles and fi- 
bres [26] . Pioneering work 
performed with 15 kV and 22 
kV polyethylene insulated ca- 
bles reported that trees ori- 
ginated from contaminants and 
cavities, the tendency for 
tree initiation from a con- 
taminant being probably more 
affected by the contaminant 
material than by the size, 
location or shape of the con- 
taminant [27] . The importance 
of enclosed cavities in the 
initiation of trees was also 
reported more recently [28] . 
According to Ieda [29], tree 
propagation can be induced by 
internal gas discharge in the 



tree. It is to be bore in 
mind that in numerous publi- 
cations dealing with experi- 
mental work, the electrode 
arrangement that was used was 
a needle-plane electrode ar- 
rangement, indicating again 
that an electrode arrangement 
was chosen, with pronounced 
interfaces, in order to study 
the treeing phenomenon [30] - 
[33]. 

Interfaces which may play 
a role in determining the 
breakdown strength of an in- 
sulating material need not be 
only interfaces between insu- 
lating material and metal or 
between insulating material 
and gas cavity or contami- 
nant. Differing phases may 
play also a role, as was no- 
ted in [34], where the inter- 
facial domain of crystalline 
and amorphous phases may de- 
termine the various proper- 
ties of semi-crystalline po- 
lymers, such as biaxially ori- 
entated polypropylene (BOPP). 
Stressing the importance of 
interfaces and experimenting 
with cross-linked polyethy- 
lene (XLPE) , McKean showed 
that a considerable improve- 
ment in cable breakdown can 
be achieved by impregnation 
with silicone oil or diethy- 
leneglycol. Such liquids can 
impregnate gas microspaces in 
the main insulation and thus 
result in an increase of the 
breakdown strength [35] . Si- 
milar observations were also 



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INTERFACES IN HIGH VOLTAGE ENGINEERING: A MOST IMPORTANT QUESTION 



reported with polypropylene 
and polyethylene impregnated 
with suitable dipolar liquids 
[36]. 

Interfacial breakdown was 
studied with various elec- 
trode geometries and insulat- 
ing systems consisting of pa- 
per typical for transformers 
and transformer oil [37] . It 
was reported that interfacial 
breakdown will occur if the 
paper is not carefully dried 
or if many gaseous micro- 
porosities are left in or on 
the paper. In [37], however, 
it was also noticed that us- 
ing a carefully prepared pa- 
per-oil interface structure, 
the breakdown does not neces- 
sarily take place at the in- 
terface. Similar observations 
were made more recently by 
using silicone rubber inter- 
faces, where both perpendicu- 
lar and parallel to the ap- 
plied electric field were in- 
vestigated [38] . 

Conventional paper-oil ca- 
ble insulation was studied 
quite early and the problems 
of interfaces were noted 
[39] . Alternative insulating 
systems, based mainly on po- 
lymeric materials, were pro- 
posed with considerable com- 
mercial success [40] - [43] . 
Modern cables with solid 
polymeric insulation did not 
avoid the problems of inter- 
faces, namely those of extru- 
sions of semi-conducting 
sheaths with the main insula- 



tion or the inclusion of mi- 
crocavities and/or impurities 
[44], [45]. Extruded cable in- 
sulation exposed to wet con- 
ditions suffered from elec- 
trochemical treeing and impu- 
rities greatly deteriorated 
its electrical performance 
[46] . Moreover, operating elec- 
trical stresses may also cau- 
se premature insulation fai- 
lure in 15 kV polyethylene 
cables, if combined with un- 
favorable interface profiles 
and moisture [47] . Interfaces 
between polyethylene and 
small contaminants or micro- 
cavities may cause bow-tie 
trees in polyethylene cables 
[48]. 

Interfaces either perpen- 
dicular to the applied elec- 
tric field or parallel to it 
or at an angle with it were 
dealt with in [49], where it 
was noted that such a variety 
of interfaces may be encoun- 
tered in applications, such 
as capacitors, cables and in 
transformer windings. Compo- 
site insulating systems must 
preserve low dielectric los- 
ses. Higher dielectric losses 
may imply high ionic concen- 
tration in a solid/liquid in- 
sulating system, i.e. high 
ionic concentration in the 
liquid component of such a 
system [50] . 

Before concluding this 
section, it is fitting to men- 
tion the composite insulating 
systems of electrical machi- 



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FUNKTECHNIKPLUS # JOURNAL 



M.G. DANIKAS, R. SARATHI 



nes, which consist mainly of 
epoxy resin and mica sheets. 
Previous work done in this 
context indicated that 
electrical treeing propagates 
through the epoxy resin and 
generally stops at the mica 
sheets, as mica is harder and 
electrically stronger than 
epoxy resin. The importance 
of such interfaces was re- 
ported before using a needle- 
plane electrode arrangement 
[51], where experiments were 
carried out without and with 
a mica sheet inserted in 
epoxy resin (Figs. 1 and 2). 
Evidently electrical trees 
were propagating more easily 
in the case of absence of the 
mica sheet and with much more 
difficulty with mica sheet. 

Simulation work done re- 
cently showed that mica she- 
ets prevent electrical trees 
from reaching the opposite 
electrode [52] . The purpose 
of mentioning experimental 
results regarding the compo- 
site system of epoxy resin/ 
mica sheets is to show that 
the electrical trees propa- 
gate through the electrically 
weaker insulating medium. The 
simulation results indicate 
that even the slightest vari- 
ations of dielectric constant 
may cause the electrical tree 
growth. In other words, the 
simulation data indicate that 



local fluctuations of dielec- 
tric constant imply local - 
even microscopically minute - 
formations of interfaces, 
which in turn may mean local 
field intensifications, en- 
couraging thus the growth of 
electrical trees (Figs. 3 and 
4). Such observations w.r.t. 
the local variations of die- 
lectric constant have also 
been reported for polyethy- 
lene [53] , [54] . 

It is evident from all the 
above that interfaces in 
classical insulating systems 
seem to cause problems (pos- 
sible dielectric constant 
mismatch, PD, treeing pheno- 
mena and ultimately risk of 
ultimate insulation failure). 
Due attention should be paid 
in choosing the insulating 
materials for specific appli- 
cations and to the construc- 
tion of the composite insu- 
lating system. Too many 
things depend on the quality 
of the construction of the 
interfaces [55]-[58], too ma- 
ny things that cannot be ig- 
nored. Interfaces in tradi- 
tional insulating systems are 
considered as the weak as- 
pects of such systems. Keep- 
ing this in mind and without 
exaggerating, it is not far 
from the truth if we state 
that an insulating system is 
as good as its weakest inter- 
faces . 



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INTERFACES IN HIGH VOLTAGE ENGINEERING: A MOST IMPORTANT QUESTION 




Fig. 1: Electrical tree propagation without the presence of 
mica sheet (applied voltage 28 kVrms, 50 Hz) (after [51]) 




Fig. 2: Electrical tree propagation with the presence of 
mica sheet. The mica sheet increases the propagation time of 
the tree (applied voltage 28 kVrms, 50 Hz) (after [51]) 



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FUNKTECHNIKPLUS # JOURNAL 



M.G. DANIKAS, R. SARATHI 





Fig. 3: Simulated electrical tree propagation with one mica 
sheet. Needle-plane electrode arrangement used 




Fig. 4: Further expansion of electrical trees. 
Electrical trees stop at mica sheet. 
Needle-plane electrode arrangement used 



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INTERFACES IN HIGH VOLTAGE ENGINEERING: A MOST IMPORTANT QUESTION 



M.G. DANIKAS, R. SARATHI 



It goes without say that 
this short review regarding 
classical interfaces does not 
by any means cover the whole 
subject and variety of solid 
insulating materials (for ex- 
ample, no mention in this pa- 
per was offered about the in- 
terfaces in outdoor polymeric 
insulators [59], [60] or in 
indoor polymeric insulators 
[61]). Both in the libraries 
and in the Internet, the in- 
terested reader may find pra- 
ctically tens of thousands of 
publications referring to the 
questions and problem areas 
of the solid insulating mate- 
rials and insulating systems. 
What this short review tried 
to do is to show that inter- 
faces, electric field inten- 
sifications, pre - breakdown 
phenomena (such as PD and 
electrical trees) and break- 
down mechanisms are all in- 
terwoven and interrelated. 
Having said that, the next 
question related to inter- 
faces, is whether they play 
the same detrimental role in 
the new generation of insu- 
lating materials, the nano- 
composite polymers. This is 
to be examined in the follow- 
ing section. 

Interfaces in Nanocomposite 

Polymers 

More than twenty years 
ago, nanocomposite polymers 
came to our lives [62] . The 
first application of nanocom- 



posites appeared in 1990, 
when Toyota Motor Corporation 
introduced nanocomposite ny- 
lon in their car industry 
[63] . Since that year, many 
car industries introduced the 
use of nanocomposite poly- 
mers. Use of nanocomposite po- 
lymers was noted in other in- 
dustries, such as in the op- 
tics and the electronics in- 
dustries as well as in the 
food industry. A seminal pa- 
per by Lewis gave the impetus 
for research also in the in- 
sulation branch [64] . 

For the electrical insula- 
tion, nanocomposite polymers 
are defined as conventional 
polymers in which particles 
smaller than 100 nm are added 
and dispersed in such a way 
that at the end one gets a 
homogeneous mixture [63] . The 
addition of such nanoparti- 
cles (the term "nanofillers" 
is also widely used) is being 
done in very small quanti- 
ties, usually less than 10 wt%. 
Nanocomposite polymers con- 
sist of three components: 

a) the base polymer (or poly- 
mer matrix), 

b) the nanoparticles (or na- 
nofillers) and 

c) the interaction zone (or 
interface zone) between the 
matrix and the nanoparticles 
[63]. 

Regarding the polymers used, 
these may be either ther- 
moplastics, thermosettings or 



elastomers. Nanoparticles may 
be classified w.r.t. their 
dimensions, and they can be 
distinguished as 

1) mono-dimensional (i.e. ex- 
tremely thin), 

2) two-dimensional (nanotu- 
bes) and 

3) three-dimensional (inorga- 
nic oxides) . 

The most usual nanoparticles 
for the purposes of electri- 
cal insulation are 

1) silica nanoparticles Si 02 , 

2) montmorillonite nanopar- 
ticles (layered silica), 

3) metallic oxides such as 
AI 2 O 3 , Ti0 2 , MgO and ZnO and 

4) carbon nanotubes. 

Nanocomposite polymers can be 
obtained in two types of 
structures, namely, 

(i) intercalated nanocompo- 
sites (formed when there is 
limited inclusion of polymer 
chain between the clay layers 
with a corresponding small 
increase in the interlayer 
spacing of a few nanometers 
and 

(ii) exfoliated nanocomposi- 
tes (formed when the clay 
layers are well separated from 
one another and individually 
dispersed in the continuous 
polymer matrix [65], [66]. 

As mentioned above, nano- 
particles are added and dis- 
persed in relatively small 



quantities in the base poly- 
mer (usually no more than 10 
wt%) . Since nanoparticles are 
smaller than microparticles 
(smaller by three orders of 
magnitude), their interaction 
with the surrounding polymer 
matrix is much greater [62] . 
The so-called interaction zo- 
ne is the main factor con- 
tributing in the improvement 
of the properties of the base 
polymer [67] . In the case of 
addition of nanoparticles in- 
to a polymer, the interfaces 
are far more numerous and far 
larger than in the case of 
microparticles. As the size 
of the added particles is re- 
duced, the interface becomes 
larger and larger. The dis- 
tance between the nanoparti- 
cles is also extremely small. 
It seems that interfaces de- 
termine to a great extent the 
properties of nanocomposite 
polymers . 

The size of nanoparticles 
and the distance between them 
is of the order of magnitude 
on nanometers. Such particles 
may interact with the polymer 
matrix both physically and 
chemically in the nanometer 
scale. This has as conse- 
quence the appearance of pro- 
perties that are somehow dif- 
ferent from those we already 
know in a more macroscopic 
scale [68] . In contradistinc- 
tion to the interfaces in 
classical insulating materi- 
als, and also to what we know 



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M.G. DANIKAS, R. SARATHI 



from classical high voltage 
textbooks, the improved insu- 
lating properties of the na- 
nocomposite polymers are due 
to 

a) the large surface area of 
nanoparticles, which creates 
a large interaction zone, 

b) the changes in polymer 
morphology because of the 
large interaction zone, 

c) the changes in the space 
charge distribution and 

d) a dispersion mechanism 
[69]- 

Both the size of the nanopar- 
ticles and the chemical pro- 
perties of their surface play 
an important role in deter- 
mining the electrical, ther- 
mal and mechanical properties 
of nanocomposites. Needless 
to say that the chemical com- 
patibility between the intro- 
duced nanoparticles and the 
polymer matrix is of para- 
mount importance for the ge- 
neral properties of the nano- 
composite [70] . 

One of the most signifi- 
cant characteristics of nano- 
composite polymers is the in- 
crease of their breakdown 
strength as the size of the 
added nanoparticles tends to 
extremely small values. This 
increase is not in agreement 
with the conventional wisdom, 
which suggests that as the 
number of interfaces in- 
creases, the breakdown stren- 
gth decreases dramatically 



[2], [3]. Nanocomposite poly- 
mers seem not to agree with 
what we already know for 
classical insulating materi- 
als or systems [71] . Diffe- 
rences in breakdown strength 
between conventional epoxy 
resin and epoxy resin with 
nanoparticles was reported in 
[72] . Such observations were 
also noted later, when six 
different materials based on 
epoxy resin with various with 
and/or micro- and nanoparti- 
cles of alumina/silica were 
tested. It was shown that 
epoxy resin with nanosilica 
particles was the most suit- 
able to obtain high values of 
breakdown strength [73] . 

Addition of the percentage 
of nanoparticles to epoxy 
resin up to a certain level 
favors the increase of break- 
down strength both with a.c. 
and d.c. voltage, as was no- 
ticed in [74] . Why nanoparti- 
cles act in such a favorable 
way, despite the numerous in- 
terfaces they create? The in- 
crease of the breakdown 
strength may be due to 

(i) the increase of the sur- 
face area of the interfaces, 
which somehow alters the be- 
havior of the polymer, 

(ii) the changes of space 
charge distribution inside 
the insulation structure, 

(iii) the dispersion mecha- 
nism, and 

(iv) the changing properties 
of the insulating material, 



more specifically its volume 
resistivity, its tan5 and its 
dielectric constant. 

It is possible that the elec- 
trons moving in such a nano- 
composite polymer, loose 
their kinetic energy because 
of the nanoparticles. Since 
the distances between the na- 
noparticles are extremely 
small, the electrons cannot 
acquire enough speed so that 
they can contribute to the 
breakdown process. Consequen- 
tly, epoxy resin with nano- 
particles presents a higher 
breakdown strength than con- 
ventional epoxy resin [74] . 

Normally the introduction 
of particles in polymeric ma- 
terials has as result the in- 
troduction of defects and 
subsequently the worsening of 
its electrical properties. 
Nanocomposite polymers seem 
not to obey the above rule, 
as the mechanisms of conduc- 
tivity during the breakdown 
process are influenced from 
the applied electric field, 
the dielectric constant of 
the nanoparticles and their 
number. The combined effect 
of these factors is difficult 
to fully understand at this 
stage and we need more work 
[71] . Similar results were 
obtained with epoxy resin 
with nanoparticles of Ti 02 as 
it presented a much higher 
breakdown strength than con- 
ventional epoxy resin [75] . 



On the other hand, elec- 
trical treeing propagation 
was found to be easier in 
conventional polymers than in 
nanocomposite polymers [76], 

[77] . Even a small wt% addi- 
tion of nanoparticles affects 
in a positive way the elec- 
trical treeing resistance of 
the nanocomposite polymer. It 
seems that electrical tree 
propagation paths go through 
the base polymer and around 
the nanoparticles (experimen- 
tal evidence for this was 
presented in SEM photographs 
published in [76]). Conse- 
quently, the more the nano- 
particles in a polymer, the 
more difficult the formation 
of treeing paths. It seems 
that nanoparticles act as ex- 
tremely small barriers, thus 
preventing the easy growth of 
electrical trees. Electrical 
trees propagate through the 
base polymer (in other words 
through the polymer matrix) 
and not through the nanopar- 
ticles. In some cases the 
electrical trees stop at the 
nanoparticles and they do not 
progress any further. Such 
observations were made in 
simulation studies recently 

[78] - [80] . 

Further research showed 
that a small percentage in- 
troduction of nanoparticles 
into a conventional polymer 
may increase its resistance 
to electrical treeing [81] . 
It is interesting to note 



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M.G. DANIKAS, R. SARATHI 



that nanoparticles may func- 
tion as barriers preventing 
the tree growth even in 
minute quantities [82] . Ear- 
lier work indicated that as 
soon as the electrical tree 
tou-ches the nanoparticle, 
the physico-chemical proper- 
ties of it are such that very 
high energies are required in 
order to cause its deteriora- 
tion [83]. Although the lat- 
ter paper is an old one, it 
may give a clue as to why 
nanoparticles act as elemen- 
tary barriers and why they 
prevent (or they delay) elec- 
trical treeing. More re- 
cently, a similar argument 
was given by some Japanese 
researchers [84] . 

Loading (i.e. the percent- 
age of included nanoparticles 
expressed in wt%) plays also 
a vital role in determining 
the resistance to electrical 
treeing. More loading (i.e. 
more nanoparticles, that is 
more interfaces) implies a 
better resistance to treeing. 
This is probably because 
trees interact with many more 
nanoparticles and this delays 
their growth [85] . Another 
possible explanation was of- 
fered some years ago, where 
the authors proposed that in 
front of a tree a damage 
process zone is formed in a 
conventional polymer. Such a 
zone cannot progress easily 
when it meets nanoparticles 
[86] . Simulation data indi- 



cated that loading affects 
the tree growth. The nanopar- 
ticle size plays also a role 
in delaying tree propagation, 
smaller nanoparticles offer a 
better tree resistance than 
the larger ones [79] . In 
Figs. 5-8 simulation results 
regarding the loading of na- 
noparticles as well as the 
size of nanoparticles are 
shown. It is evident that mo- 
re loading makes tree growth 
more difficult. It goes also 
without say that smaller 
nanoparticles offer a better 
resistance to tree propaga- 
tion . 

There is no need to empha- 
size that there is also in 
the field of nanocomposite 
polymers a vast body of tech- 
nical literature, too vast to 
be mentioned here in the con- 
text of this paper. From this 
short review it is obvious 
that interfaces in nanocom- 
posites play an entirely dif- 
ferent role from the one they 
play in classical insulating 
materials. Why is this so? 
This may be because the phy- 
sics and/or chemistry somehow 
change in the nanoscopic 
world of such materials. In- 
terfaces become highly desir- 
able - at least up to a cer- 
tain percentage of added na- 
noparticles. The surface area 
of the nanoparticles is huge 
if compared with that of mi- 
croparticles for other high 
voltage applications. 




Fig. 5: Simulation with epoxy resin filled with TiC >2 nano- 
particles (loading of 2 wt%, nanoparticle diameter 100 nm) 




Fig. 6: Simulation with epoxy resin filled with Ti02 nano- 
particles (loading of 6 wt%, nanoparticle diameter 100 nm) 



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INTERFACES IN HIGH VOLTAGE ENGINEERING: A MOST IMPORTANT QUESTION 



M.G. DANIKAS, R. SARATHI 




Fig. 7: Simulation with epoxy resin filled with TiC >2 nano- 
particles (loading of 6 wt%, nanoparticle diameter 200 nm) 




Fig. 8: Simulation with epoxy resin filled with Ti02 nano- 
particles (loading of 6 wt%, nanoparticle diameter 100 nm) 



Models regarding the ex- 
planation of the functioning 
of nanocomposites were pro- 
posed as well as how the 
nanoparticles behave inside 
the base polymeric material 
[62], [87], [88]. Such models 
try to explain the higher 
breakdown strength values of 
nanocomposites and also their 
higher resistance to electri- 
cal treeing. The explanations 
seem to be plausible but more 
hard evidence is needed. Such 
evidence will be offered by 
many more photographs (SEM 
and TEM photos) showing in 
detail how the electrical 
trees circumvent the nanopar- 
ticles and how they propagate 
through the polymer matrix. 
Such photographs are very 
difficult to obtain. 

Where all this leaves us? 
How can we understand in a 
unified way interfaces in 
both classical and nanocom- 
posite insulating systems? 
Can the experimental results 
and simulations on electrical 
treeing with typical machine 
insulation ([52]) provide a 
hint also for a possible ex- 
planation in the nanoscopic 
world? A recent paper on in- 
terfaces posed some pertinent 
questions regarding classical 
and nanocomposite insulating 
materials [89] . From this pa- 
per it is obvious that al- 
though more questions are in 
need of an answer for the na- 
nocomposites, the subject of 



interfaces in classical insu- 
lating systems is by no means 
finished. For example, charg- 
ing of larger interfaces, 
such as found in cable joints 
and terminations, needs to be 
further explored regarding 
the mechanisms of space char- 
ges. Another aspect in need 
of further discussion is whe- 
ther electrical trees may em- 
anate from enclosed cavities 
in conventional polymers. The 
latter question has been par- 
tly answered in [28], [90] - 
where some experimental evi- 
dence was offered as to the 
possibility of electrical 
trees stemming from enclosed 
cavities - but further expe- 
rimental data is needed. 

One thing that should not 
be forgotten - and it is com- 
mon to conventional as well 
as nanocomposite polymers - 
is that an insulating system 
to a great extent is as good 
as its interfaces. This means 
that, no matter whether we 
deal with conventional insu- 
lating materials or nanocom- 
posites, preparation and con- 
struction in both cases has 
to be not only careful but 
meticulous . 

A last remark on the li- 
terature presented here: the 
interested reader may find 
that the authors dwell per- 
haps too much in the older 
scientific literature. This 
is not done because they tend 
to ignore the more recent re- 



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M.G. DANIKAS, R. SARATHI 



search: they simply would 
like to show that even in the 
old days, the problems were 
more or less the same. More- 
over, they would also like to 
show that fundamental ideas - 
which are with us even today 
- on the various mechanisms 
in insulating materials came 
about quite early. 

Conclusion 

This review - by no means 
exhaustive - tackled the sub- 
ject of the importance of in- 
terfaces both in conventional 
insulating materials and in 
nanocomposite polymers. Whe- 
reas interfaces are to be 
avoided in conventional mate- 
rials, they seem to be a 
blessing in nanocomposites. 



Whereas in conventional mate- 
rials they cause problems of 
compatibility and sometimes 
high field intensifications 
with all the bad consequences 
such intensifications entail 
(i.e. PD, trees), in nanocom- 
posites they seem - up to a 
certain loading - to be de- 
sirable and they prevent (or 
the delay) tree growth. Whe- 
reas in conventional insulat- 
ing systems the introduction 
of more interfaces seems to 
cause sometimes insurmount- 
able problems, the introduc- 
tion of interfaces (because 
of the nanoparticles' intro- 
duction to the polymer ma- 
trix) seems to alleviate elec- 
trical trees and to distrib- 
ute more evenly electric 
fields and space charges. 



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* About The Authors 

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FRONT COVER VIGNETTE 

A faded synthesis of an anthemion rooted in a meandros 

The thirteen-leaf is a symbol for a life tree leaf. 

"Herakles and Kerberos", ca. 530-500 BC, 
by Paseas, the Kerberos Painter, 
Museum of Fine Arts, Boston. 

www. mfa . org/collect ion s/object /plate -153852 

The simple meandros is a symbol for eternal immortality. 

"Warrior with a phiale", ca. 480-460 BC, 

by Berliner Maler, 

Museo Archeologico Regionale "Antonio Salinas" di Palermo, 
commons .wikimedia . org/wiki/File :Warrior_MAR_Palermo_NI2134. j pg